We have now accumulated a sufficient amount of evidence to implicate fibronectin as being the collagen receptor in platelets. Our evidence includes the following observations: (1) platelets contain fibronectin as shown by its precipitation by antifibronectin antibodies. (2) Fibronectin, extracted from platelets and purified, binds to collagen. (3) Anti-fibronectin antibodies and their F(ab')2 fragments stimulate platelets, whereas preimmune antibodies do not. (4) Fab' fragments of anti-fibronectin antibodies do not stimulate platelets, but do inhibit the collagen:platelet interaction when pre-incubated with platelets. They have no effect on the thrombin:platelet interaction. We are planning to extend this investigation to determine if: (1) platelet fibronectin is a component of the exterior membrane surface; (2) platelet-membrane fibronectin binds directly to collagen; (3) platelet Factor XIIIa can crosslink platelet fibronectin with collagen; and (4) fibronectin is associated with other proteins in the platelet membrane. In addition, we will attempt to determine the mechanism by which the binding of platelet fibronectin to collagen is able to cause the stimulation of the platelet.